1. Field of the Invention
The present invention relates to a transmission control method and device, and in particular to a transmission control method and device in mobile stations which mutually notify their own information by using a CSMA method without interposition of a master station in order to prevent a collision between the mobile stations.
2. Description of the Related Art
When communication is performed by the CSMA method, a mobile station has performed a carrier sense before making data transmissions, thereby confirming whether or not data from other mobile stations exists in the air to make data transmissions of its own station when determining that no data from other mobile stations exists. The transmissions have been made with the same power enough to cover a communication area determined as a system for all of the mobile stations.
While there are some CSMA methods, one example of them will now be described referring to FIG. 7 which is a diagram showing the concept and FIG. 8 which shows a data transmission flow in each mobile station. It is to be noted that while by a normal CSMA method a data receiving side returns an ACK signal for confirming that data has arrived, a broadcast communication is presumed in the case of FIG. 7, so that no ACK signal is used.
Time T1: A mobile station MS2 receives a data transmission request from an upper layer, and performs a carrier sense upon data transmission (at step S32). At this time, since a carrier is sensed due to a data transmission of a mobile station MS1, the carrier sense is continued until no carrier is sensed (at step S33).
The mobile station MS2 has determined to make transmission at the timing of a time T1 based on a result of sole determination of random transmission timing by the mobile station MS2 (at step S31). Other mobile stations may perform a carrier sense for transmission at the same timing.
Time T2: Detecting that transmission data of the mobile station MS1 has been completed (at step S34), the mobile station MS2 starts up a timer of fixed waiting time (at step S35).
Time T3: Since the fixed waiting time in the mobile station MS2 is up (at step S36), the mobile station MS2 starts up a random timer at this point (at step S37).
Time T4: Having received the data transmission request from the upper layer, the mobile station MS3 performs a carrier sense for the data transmission (at step S32). Since having found that there is no carrier as a result of the carrier sense (at step S33), the mobile station MS3 starts up the timer of fixed waiting time (at step S35).Time T5: The random time in the mobile station MS2 is up (at step S38). Then, the mobile station MS2 performs the carrier sense again (at step S39). Having detected no carrier transmitted by another mobile station (at step S40), the mobile station MS2 starts its data transmission (at step S41).Time T6: Since the fixed waiting time which started at the time T4 is up (at step S36), the mobile station MS3 starts up the random timer (at step S37).Time T7: Since the random time in the mobile station MS3 is up (at step S38), the mobile station MS3 performs the carrier sense (at step S39). In the presence of transmission data of the mobile station MS2, the mobile station MS3 detects the carrier. Then, the carrier sense is continued until no carrier is sensed (at step S33).Time T8: After the completion of the data transmission of the mobile station MS2 and the detection of absence of carrier by the mobile station MS3 (at step S34), the mobile station MS3 starts up the fixed timer again at this point (at step S35).Time T9: Since the fixed time in the mobile station MS3 is up (at step S36), the mobile station MS3 starts up the random timer (at step S37), where the maximum value of the random timer time at this time is gradually reduced in inverse proportion to the number of start-ups until the data transmission.Time T10: Since the random time in the mobile station MS3 is up (at step S38), the mobile station MS3 performs the carrier sense again (at step S39), which results in the detection of absence of carrier (at step S40), so that the data is transmitted (at step S41).
It is to be noted that there is a base station device and a transmission power control method in which the base station device is provided with a transmitting device which transmits wireless transmission data, a transmission power control circuit which controls a transmission power of the transmitting device, a receiving device which receives the wireless transmission data from a mobile device and generates reception data, and a retransmission detecting circuit which detects a retransmission request included in the reception data. The transmission power control circuit is provided with an initial transmission power controller which increases the transmission power of the transmitting device step-by-step from a transmission power initial value predetermined in an initial period of the transmission start according to the retransmission request and stops the increase of the transmission power when the retransmission detection circuit detects no retransmission request.    [Patent document 1] Japanese Patent Application Laid-open No. 2004-328394
However, when the communication according to the above-mentioned CSMA method is performed and each moving speed of the mobile stations within the same communication area is low, numerous mobile stations will concurrently exist in a single communication area. Conversely, when numerous mobile stations exist within the communication area, it is natural that the mobile stations move at a low speed. In this case, the packet occupancy in the air is increased, and “carrier presence” is indicated in almost all time zones even when the carrier sense is performed, so that a time zone during which a packet can be transmitted by its own station can not be found. Eventually, a collision between the mobile stations is induced. This will be specifically described by referring to FIGS. 9A and 9B.
It is supposed that ten mobile stations MS1-MS10 exist, and that there is only a time for transmitting transmission packets of ten mobile stations MS1-MS10 in a single frame in order to simplify the description. Also, it is supposed that transmission is being made with the maximum power so that all of the mobile stations MS1-MS10 may cover a maximum required communication area as shown in FIG. 9A.
Although the moving speeds of the mobile stations MS1-MS10 are different from each other (the moving speed of only the mobile station MS4 is high) in the prior art method, all of the mobile stations transmit with the same transmission power, so that the same communication area is formed. When a mobile station MS11 newly appears and performs a carrier sense for transmitting its own data as shown in FIG. 9B, there is found no “vacancy” in the packet occupation status in the air at the position of the mobile station MS11, and “carrier presence” is indicated, so that a transmission chance is not be provided. Accordingly, there is a possibility that a collision between the mobile stations may be induced.